Electrodeposition is widely used to coat a functional material having a desired property onto a surface that otherwise lacks that property. During electrodeposition, electrically charged reactants in an electrolyte solution diffuse, or are moved by an electric field, to cover the surface of an electrode. For example, the electrical current can reduce reactant cations to yield a deposit on an anode. Or, anions of reactants in the electrolyte solution can diffuse, or be moved by the electric field, to cover the surface of a cathode, where the reactant anions are oxidized to form a deposit on the electrode.
Electrodeposition has been successfully utilized in the fields of abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc. It also occurs in the operation of certain energy storage devices. For example, in the charge process of a metal battery or metal-ion battery, metal ions in the electrolyte move from the cathode and are deposited on the anode. Some organic compounds with unsaturated carbon-carbon double or triple bonds are used as additives in non-aqueous electrolytes and are electrochemically reduced and deposited at the anode surface or oxidized and deposited at the cathode surface to form solid electrolyte interphase layers as protection films on both anode and cathode of lithium batteries. Some other organic compounds with conjugated bonds in the molecules are electrochemically oxidized and deposited at the cathode surface to form electric conductive polymers as organic cathode materials for energy storage devices.
In most instances, the ideal is a smooth electrodeposited coating. For example, a smoothly plated film can enhance the lifetime of a film used for decoration, wear resistance, corrosion protection, and lubrication. A smoothly plated film is also required for energy storage devices, especially for secondary devices. Rough films and/or dendrites generated on electrode surfaces during the charge/discharge processes of these energy storage devices can lead to the dangerous situations, short-circuits, reduced capacities, and/or shortened lifetimes.
Roughness and/or dendrites can be caused by several reasons, including the uneven distribution of electric current density across the surface of the electrodeposition substrate (e.g., anode) and the uneven reactivity of electrodeposited material and/or substrate to electrolyte solvents, reactants, and salts. These effects can be compounded in the particular case of repeated charging-discharging cycles in energy storage devices. Therefore, a need for improved electrolytes and methods for electrodeposition are needed to enhance the smoothness of the resultant film.